The present invention relates to a room temperature-curable organopolysiloxane composition or, more particularly, to a room temperature-curable organopolysiloxane composition having an improved rheological property of thixotropy and capable of being cured at room temperature by the mechanism of deacidification into a silicone rubber which can be firmly bonded adhesively to the surface of various kinds of substrate materials such as metals and plastics on which the composition has been cured.
There are known various types of room temperature-curable or vulcanizable organopolysiloxane compositions, referred to as RTV compositions hereinbelow, capable of being cured at room temperature into a silicone rubber differing in the formulation and mechanism of the crosslinking reaction. Among the RTV compositions of different types, one of the most widely used types includes those acurable by the mechanism of a condensation reaction to produce a carboxylic acid as a byproduct and they are widely used as a caulking and sealing material, for example, in building and construction works and electric and electronic instruments.
Notwithstanding the generally very excellent properties of the RTV compositions of the deacidification type as mentioned above, the RTV compositions usually involves a problem in the adhesive bonding of the cured silicone rubber to the substrate surface on which the composition has been cured. Although considerably good adhesive bonding can be obtained between the RTV composition and the surface of a substrate of, for example, glass and ceramic materials, namely, the adhesive bonding strength can be not always satisfactorily high on to the surface of a metal or plastic resin as well as on to the surface coated with a coating composition such as a paint though dependent on the surface condition. Furthermore, even the adhesive bonding strength with a glass or ceramic material may be remarkably decreased when the bonded articles are dipped in water prolongedly. Therefore, it is usually essential to subject the surfaces to be bonded together to a treatment with a primer before adhesively bonding with the RTV composition. Even by setting aside the problem of difficulty in the appropriate selection of a primer in relation to the material of the substrate, disadvantages are unavoidable in the use of a primer that the coating works with a primer are usually very troublesome sometimes leaving uncoated areas on the substrate surface to cause uneven adhesive bonding. Moreover, certain kinds of substrate materials are subject to the attack of the organic solvent usually contained in the primer composition in the form of a solution so that cracks are formed on the substrate surface or the mechanical strength of the substrate is greatly decreased.
Accordingly, a proposal is made in Japanese Patent Publications Nos. 49-7578, 49-30693 and 55-22512 according to which a RTV composition of the deacidification type is admixed with di-tert-butoxy diacetoxy silane, 3-glycidyloxypropyl trimethoxy silane or the like as an adhesion aid. The addition of these silane compounds is, however, detrimental in respect of the rheological properties of the composition such as thixotropy though effective to some extent in respect of the improvement of the adhesive bonding. This drawback accompanying the addition of the adhesion aid can be remedied by the admixture of a thixotropy-improver though again with sacrifice of the adhesive bonding strength.